Leibfried, in U.S. Pat. Nos. 4,900,779 and 4,902,731 and U.S. patent application Nos. 07/419,429 (filed Oct. 10, 1989) (now U.S. Pat. No. 5,013,809) and 07/419,430 (filed Oct. 10, 1989) (now U.S. Pat. No. 5,077,134) and Bard and Burnier, in U.S. patent application No. 07/422,214 (filed Oct. 16, 1989), (now U.S. Pat. No. 5,008,360) describe cross-linked organosilicon polymers and cross-linkable organosilicon prepolymers comprised of polycyclic hydrocarbon residues and cyclic polysiloxanes or siloxysilane residues linked through carbon to silicon bonds, and processes useful for preparing the same. These polymers have high glass transition temperatures (T.sub.g), low dielectric constant, low moisture absorption and other desirable properties. The polymers and prepolymers are described as useful for electronic applications, such as preparing printed circuit boards (including substantially tack-free prepreg and laminates useful for preparing such circuit boards) and encapsulants, and structural materials.
One major weakness of these polymers is their brittleness. Brittleness can result in cracking or poor adhesive strength (e.g., poor adhesion of copper foil to circuit board laminate).
The inventors have discovered that certain elastomers improve the copper peel strength of and toughen the polymers and prepolymers, without significantly impacting other properties, such as Tg.